The present invention relates to a device for supplying to electric power to a superconductor, in particular to a superconducting winding, which can be cooled to a predetermined temperature for achieving superconductivity.
A superconducting rotor with a superconducting winding for an electric motor is known, for example, from U.S. Pat. No. 5,482,919 A. The superconducting winding is cooled by a cooling system to a sufficiently low temperature, so that the coil becomes superconducting. Preferably, a high-temperature superconducting material (HTSC) is used with a superconducting transition temperature above approximately 35 K. The coil can be cooled by a cooling system that employs the Gifford-McMahon cycle or the Stirling cycle process for cooling.
An AC current is supplied to the superconducting coil via brush rings. However, the brushes can wear down which adversely affects the life expectancy of the motor, and the brush rings can introduce excessive heat into the cooled region. For example, when the current is supplied through a current supply line that is cooled only by thermal conduction, approximately 45 W/kA are introduced into the cooled region at a temperature of between 20 K and 40 K. At least 27 W of thermal energy are introduced by the two required current supply lines at a typical operating current in the superconductor of, for example, 300 A. This approximately equals the total cooling power of a conventional high-efficiency Gifford-McMahon cooler (approximately 25 W at 20 K). The operating temperature of the coil increases with the heat loss introduced by the current supply lines. This reduces the critical current of the superconductor and hence also the magnetic field strength attained by the coil. This limits the current that can be efficiently supplied to the motor and makes the operation of the motor less cost-effective.
It would therefore be desirable to provide a device for supplying electric power to a superconductor, in particular to a superconducting winding of a motor, to obviate prior art shortcomings and to operate efficiently and economically. It would also be desirable to minimize heat transfer via the current supply lines, so that the superconductive properties of the superconductor and hence the magnetic field strength of the superconducting winding or coil can be maintained.